1. Field of the Invention
This invention relates to circuit interrupters provided with both ground fault and arc fault trip mechanisms and, more particularly, to a circuit breaker having a test actuator for enabling ground fault and arc fault tests.
2. Background Information
Circuit interrupters include, for example, circuit breakers, contactors, motor starters, motor controllers, other load controllers and receptacles having a trip mechanism. Circuit breakers are generally old and well known in the art. Examples of circuit breakers are disclosed in U.S. Pat. No. 5,260,676; and U.S. Pat. No. 5,293,522.
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal which is heated and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system. An armature, which is attracted by the sizable magnetic forces generated by a short circuit or fault, also unlatches, or trips, the operating mechanism.
In many applications, the miniature circuit breaker also provides ground fault protection. Typically, an electronic circuit detects leakage of current to ground and generates a ground fault trip signal. This trip signal energizes a shunt trip solenoid, which unlatches the operating mechanism, typically through actuation of the thermal-magnetic trip device.
A common type of ground fault detection circuit is the dormant oscillator detector including first and second sensor coils. The line and neutral conductors of the protected circuit pass through the first sensor coil. The output of this coil is applied through a coupling capacitor to an operational amplifier followed by a window comparator having two reference values. A line-to-ground fault causes the magnitude of the amplified signal to exceed the magnitude of the reference values and, thus, generates a trip signal. At least the neutral conductor of the protected circuit passes through the second sensor coil. A neutral-to-ground fault couples the two detector coils which causes the amplifier to oscillate, thereby resulting in the generation of the trip signal. See, for example, U.S. Pat. No. 5,260,676; and U.S. Pat. No. 5,293,522.
Recently, there has been considerable interest in also providing protection against arc faults. Arc faults are intermittent high impedance faults which can be caused, for instance, by worn insulation between adjacent conductors, by exposed ends between broken conductors, by faulty connections, and in other situations where conducting elements are in close proximity. Because of their intermittent and high impedance nature, arc faults do not generate currents of either sufficient instantaneous magnitude or sufficient average RMS current to trip the conventional circuit interrupter. Even so, the arcs can cause damage or start a fire if they occur near combustible material. It is not practical to simply lower the pick-up currents on conventional circuit breakers, as there are many typical loads which draw similar currents and would, therefore, cause nuisance trips. Consequently, separate electrical circuits have been developed for responding to arc faults. See, for example, U.S. Pat. No. 5,224,006; and U.S. Pat. No. 5,691,869.
Ground fault protection circuits and arc fault protection circuits typically include separate associated test circuits for affirming their continued operability.
Currently, separate test switches are provided for performing the ground fault and arc fault tests. See, for example, U.S. Pat. No. 5,459,630.
For ground fault circuit interrupters (GFCIs), the UL standard mandates that the device (e.g., receptacle, circuit breaker) includes a test switch which activates ground fault protection circuitry to detect line-to-ground and neutral-to-ground faults. In turn, the user may periodically check the operation of the device, by employing the test switch to cause the separation of the separable contacts.
For arc fault circuit interrupters (AFCIs), the draft UL standard proposes testing of the arc fault detection function.
Although multiple waveform tests could be achieved by multiple individual test switches, with operation of any individual test switch resulting in waveform analysis which, if successful, would cause separable contact opening, there is room for improvement.
There is also a more particular need for a circuit breaker providing ground fault and arc fault protection which has an improved mechanism for testing both the ground fault detector and the arc fault detector and, yet, can be contained in the standardized molded cases of miniature circuit breakers for use with conventional load centers.